On the one hand, the invention relates to a shed forming device, comprising means provided to position warp yarns in order to form a shed in accordance with a pattern to be woven before inserting a weft through this shed, the positioning of the warp yarns being the possible cause of an increase or a decrease of potential energy in the shed forming device. On the other hand, the invention relates to a weaving machine provided with such a shed forming device.
In shed forming devices according to the state-of-the-art, potential energy is built up on the basis of warp yarn tension, this potential energy being possibly additionally supplied with the potential energy of spring systems serving to bring the warp yarns back in a specific position. In accordance with the pattern to be woven, the warp yarns are moved, individually or as a group from one position to another position, whereby the warp yarn tension and/or the tension of the springs will be varied. Each time, this leads to a modified situation of the potential energy, which, in case of an excess, will be lost in the form of heat, and which, in case of want, has to be supplemented by supplying energy through the driving device. Forming the weaving pattern required may cause strong variations in the supply of energy by the driving device. Supply or removal of energy has to be performed during each machine cycle. Therefore, in case of strong variations it may occur that each machine cycle will need a strong variation of the energy supply. The operational speeds of such machines may amount to a 1000 cycles per minute or more.
Driving systems of, among others, weaving machines, Jacquard machines and Jacquard weaving machines are not the right systems to supply such short variations of the energy supply, without causing an irregular behavior of the driving device which, in turn, means an irregular load on the components of the driving device and a variable operational speed. This can be the possible cause of an irregular quality of the fabrics produced or may even cause machine parts to be broken in the cinematic chain of the weaving machines or Jacquard machines.
Until recently, in flat Jacquard weaving, the quality of the most frequent fabric structures was that the number of warp yarns going up and the number of warp yarns going down, most of the time, were almost the same. In this manner the variation in potential energy occurring, was limited and therefore the load on the Jacquard drive was practically a constant one. With new weave structures this is not always the case any longer and a growing number of variations has come up, giving cause to an irregular demand of energy supply and an irregular progress of the speed.
When weaving face-to-face carpets, where the non-active pile warp yarns (the dead pile warp yarns) are woven into the backing fabric, it sometimes happens that the dead pile warp yarns are sequentially selected, parting from the outside of the inner shot to the inside of the outer shot, where in quite a number of weaves, these position changes will occur synchronically between the upper and the lower fabric, in other words, the dead pile is moving up simultaneously and moving down simultaneously, both in the upper and in the lower fabric. This will cause an unbalanced building up of potential energy, resulting in an irregular load of the Jacquard machine as a function of the time.
These effects likewise occur with other shed forming devices, such as selvedge systems and shaft machines.
With selvedge systems, the number of warp yarns involved is smaller, but the drive of the selvedge system is lighter in the same proportion. Finally, the effect of an irregularly varying potential energy is as problematic as with Jacquard machines.
With shaft machines, each selection element drives a shaft that is moving a set of warp yarns. Here also, the number of shafts is limited, nevertheless the influence of the variation of potential energy is considerable because of the large number of warp yarns, the potential energy of which is changing per motion of a shaft. With a fabric structure in which the number of shafts moving up is different from the number of shafts moving down, the fluctuations in potential energy are considerable and the result is an operation because of which the weaving machine will function at an uncontrolled variable speed. In EP 0 860 528 a solution is proposed to render variable the spring force of the retracting springs that is caused by the various positions taken up by the warp yarns. Because the heddles and warp yarns are drawn downwards under a constant pressure by means of a cylinder, the force exerted on the heddles and on the warp yarns remains practically constant, consequently the Jacquard device will no longer have to compensate multiples of the retracting force as the Jacquard machine will attain more positions. Moreover, the retracting force can be made adjustable in a simple way, according to the invention, especially by adapting the operating pressure of the device.
However, a disadvantage of this device is that the effect will not present a solution for an unbalanced load of a Jacquard device, as may occur, for instance, because of the dead pile warp yarns that are moving synchronically in the upper and lower fabric.
In EP 0 374 279 a solution is proposed to reduce the heavy load on hooks, for instance when the installation is operating with repeats, and therefore the hook will have to absorb many times the force of the retracting spring. By using springs operating above the point where the repeats are disconnected, the increased load on the hooks is partly compensated. By installing the springs to be movable, the spring force can be adjusted as a function of the fabric or as a function of the repeat changing.
Nor is this possibility to compensate proposing any possibility whatsoever to compensate the irregular load caused by the pattern of the weave, as all the hooks are compensated in the same way independently of the load caused by the pattern.
In EP 0 529 025 a preferred embodiment is described in which a spring is provided that is connected to a tackle element in order to reduce the resulting force required by the retracting spring. Here also, an effect is realized by which the load on the retracting springs is reduced so that the dimensions of the retracting springs can be reduced, or a longer life span can be obtained. Neither will this solution enable to compensate the irregularities of the load, as a function of the pattern.
In U.S. Pat. No. 5,462,093 a system is described for damping the oscillating motion of a weaving machine by means of providing tension or compression springs, in order to obtain a state of equilibrium.
The disadvantage of such an adjustment is that it only proposes a solution within a certain speed range and it requires special provisions to be able to supply additional energy when starting and at low speeds. Neither will this damping enable a compensation for the unbalance linked to the pattern.